Operating parameters considered are in particular operating voltage or voltage supply and/or the operating temperature of integrated circuits.
In order to allow for safe operation of integrated circuits (ICs), thresholds on operating parameters such as supply voltage and/or operating temperature have to be met. If for example the supply voltage falls below or exceeds the admissible or predetermined limit, error-free operation of the IC is no longer ensured. If a defined condition is to be achieved even with a too low supply voltage, a voltage monitor IC is typically used, which monitors the voltage and generates a reset signal when the voltage drops below a predefined threshold.
As voltage supplies become smaller and smaller and have smaller and smaller allowable tolerances, monitoring thereof becomes increasingly difficult and costly. In some cases voltage monitors are used which generate a reset signal only beyond the permissible supply voltage and thus leave small supply voltage ranges in an undefined state.
Voltage monitors commonly employed comprise analog circuits for monitoring the supply voltage. Also known are voltage monitors that are implemented in digital ICs. However, it is more difficult to additionally integrate analog circuits into a digital IC. Accordingly, the integrated voltage monitors are very imprecise and leave undefined a very wide voltage range.
To give an example of the operating voltage of an integrated circuit, the core voltage for the FPGA Cyclone III from Altera (www.altera.com) may be cited. According to the data sheet (manufacturer's information) the admissible range is:                Maximum: 1.25 V        Typical: 1.2 V        Minimum: 1.15 V        
A typical triggering threshold of the integrated voltage monitor is V=0.77 V. Thus, there is an undefined range from 0.77 V to 1.15 V. In this range, the behavior of the IC is undefined. It cannot be assumed with confidence that the circuit or the FPGA performs its function.
In addition, any voltage monitors increase the cost of a circuit. Especially in case of high volumes, significant cost savings could be realized if external or internal voltage monitors could be dispensed with.
A further disadvantage is that the voltage monitors occupy real estate on a circuit board, printed circuit board or chip surface. Though chip surface is very precious in general, the voltage monitor's surface requirements, moreover, are in contradiction to the general trend towards miniaturization.
Furthermore, the voltage monitors need to be adapted to or calculated for the circuit. Voltage monitors having an undefined range smaller than that indicated in the example above have higher tolerance requirements which increase manufacturing costs.